How to "Un-Eject" a USB Flash drive -- Eject is easy (safely remove hardware), but what about Un-Eject or Reconnect? [duplicate]
You need to remount the drive. Depending on the operating system there are several ways to do this.
Assuming Windows:
Right click My Computer and select Properties (or you can simultaneously press Win+Pause/Break key). In System Properties window, go to the Hardware tab and then click the Device Manager button. In Windows Vista or Windows 7, you can just click the Device Manager link located at the left hand sidebar when you are at the System Properties window.
Click View from the menu bar and select “Devices by Connection“.
Find the parent USB Root Hub of the USB device that has a yellow color exclamation mark. Right click on the USB Root Hub and select Disable. Again right click on the same USB Root Hub that you’ve just disabled and this time select Enable.
From Remount Ejected or Safely Removed Drive.
A little Google-Fo will yield results for OS X and various Linux distro's.